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<h2><span class="refentrytitle">Function template let</span></h2>
<p>boost::xpressive::let — For binding local variables to placeholders in semantic actions when constructing a <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1regex__iterator.html" title="Struct template regex_iterator">regex_iterator</a></code> or a <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1regex__token__iterator.html" title="Struct template regex_token_iterator">regex_token_iterator</a></code>. </p>
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<h2 class="refsynopsisdiv-title">Synopsis</h2>
<div class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../xpressive/reference.html#doxygen.regex__actions_8hpp" title="Header &lt;boost/xpressive/regex_actions.hpp&gt;">boost/xpressive/regex_actions.hpp</a>&gt;

</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span><span class="special">...</span> ArgBindings<span class="special">&gt;</span> <span class="emphasis"><em><span class="identifier">unspecified</span></em></span> <span class="identifier">let</span><span class="special">(</span><span class="identifier">ArgBindings</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">...</span> args<span class="special">)</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.44.5.4.49.4"></a><h2>Description</h2>
<p>
<code class="computeroutput"><a class="link" href="namespaceboost_1_1xpressive_1ace777d41d9a728658b3569d818e70d52.html" title="Function template let">xpressive::let()</a></code> serves the same purpose as <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1match__results.html#doxygen.structboost_1_1xpressive_1_1match__results_1a0b5d9b054f18782e2e195dab5e39123a">match_results::let()</a></code>; that is, it binds a placeholder to a local value. The purpose is to allow a regex with semantic actions to be defined that refers to objects that do not yet exist. Rather than referring directly to an object, a semantic action can refer to a placeholder, and the value of the placeholder can be specified later with a <span class="emphasis"><em>let expression</em></span>. The <span class="emphasis"><em>let expression</em></span> created with <code class="computeroutput"><a class="link" href="namespaceboost_1_1xpressive_1ace777d41d9a728658b3569d818e70d52.html" title="Function template let">let()</a></code> is passed to the constructor of either <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1regex__iterator.html" title="Struct template regex_iterator">regex_iterator</a></code> or <code class="computeroutput"><a class="link" href="structboost_1_1xpressive_1_1regex__token__iterator.html" title="Struct template regex_token_iterator">regex_token_iterator</a></code>.</p>
<p>See the section  <a class="link" href="../xpressive/user_s_guide.html#boost_xpressive.user_s_guide.semantic_actions_and_user_defined_assertions.referring_to_non_local_variables"> "Referring to Non-Local Variables"</a> in the Users' Guide for more discussion.</p>
<p><span class="emphasis"><em>Example:</em></span> </p>
<pre class="programlisting"><span class="comment">// Define a placeholder for a map object:</span>
<a class="link" href="structboost_1_1xpressive_1_1placeholder.html" title="Struct template placeholder"><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span></a> <span class="special">&gt;</span> <span class="identifier">_map</span><span class="special">;</span>

<span class="comment">// Match a word and an integer, separated by =&gt;,</span>
<span class="comment">// and then stuff the result into a std::map&lt;&gt;</span>
<a class="link" href="structboost_1_1xpressive_1_1basic__regex.html" title="Struct template basic_regex"><span class="identifier">sregex</span></a> <span class="identifier">pair</span> <span class="special">=</span> <span class="special">(</span> <span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1ad80f6f2e945d16adb777f6c5c05c32ba.html" title="Global s1"><span class="identifier">s1</span></a><span class="special">=</span> <span class="special">+</span><a class="link" href="namespaceboost_1_1xpressive_1a6d4422b6535a447d3eb0689827b0b8fe.html" title="Global _w"><span class="identifier">_w</span></a><span class="special">)</span> <span class="special">&gt;&gt;</span> <span class="string">"=&gt;"</span> <span class="special">&gt;&gt;</span> <span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1a1f240bb30623f86f70e07f05bbd7b086.html" title="Global s2"><span class="identifier">s2</span></a><span class="special">=</span> <span class="special">+</span><a class="link" href="namespaceboost_1_1xpressive_1afae3ed7c3a8ebd3c2f06a943ff974a58.html" title="Global _d"><span class="identifier">_d</span></a><span class="special">)</span> <span class="special">)</span>
    <span class="special">[</span> <span class="identifier">_map</span><span class="special">[</span><a class="link" href="namespaceboost_1_1xpressive_1ad80f6f2e945d16adb777f6c5c05c32ba.html" title="Global s1"><span class="identifier">s1</span></a><span class="special">]</span> <span class="special">=</span> <a class="link" href="namespaceboost_1_1xpressive_1a149e8cb8ad542bdabd5c1980bb584c21.html" title="Function template as"><span class="identifier">as</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></a><span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1a1f240bb30623f86f70e07f05bbd7b086.html" title="Global s2"><span class="identifier">s2</span></a><span class="special">)</span> <span class="special">]</span><span class="special">;</span>

<span class="comment">// The string to parse</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <a class="link" href="namespaceboost_1_1xpressive_1a3359fafa9e5c7964a0bd5364de930e9c.html" title="Global str"><span class="identifier">str</span></a><span class="special">(</span><span class="string">"aaa=&gt;1 bbb=&gt;23 ccc=&gt;456"</span><span class="special">)</span><span class="special">;</span>

<span class="comment">// Here is the actual map to fill in:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">result</span><span class="special">;</span>

<span class="comment">// Create a regex_iterator to find all the matches</span>
<a class="link" href="structboost_1_1xpressive_1_1regex__iterator.html" title="Struct template regex_iterator"><span class="identifier">sregex_iterator</span></a> <span class="identifier">it</span><span class="special">(</span><a class="link" href="namespaceboost_1_1xpressive_1a3359fafa9e5c7964a0bd5364de930e9c.html" title="Global str"><span class="identifier">str</span></a><span class="special">.</span><span class="identifier">begin</span><span class="special">(</span><span class="special">)</span><span class="special">,</span> <a class="link" href="namespaceboost_1_1xpressive_1a3359fafa9e5c7964a0bd5364de930e9c.html" title="Global str"><span class="identifier">str</span></a><span class="special">.</span><span class="identifier">end</span><span class="special">(</span><span class="special">)</span><span class="special">,</span> <span class="identifier">pair</span><span class="special">,</span> <a class="link" href="namespaceboost_1_1xpressive_1ace777d41d9a728658b3569d818e70d52.html" title="Function template let"><span class="identifier">let</span></a><span class="special">(</span><span class="identifier">_map</span><span class="special">=</span><span class="identifier">result</span><span class="special">)</span><span class="special">)</span><span class="special">;</span>
<a class="link" href="structboost_1_1xpressive_1_1regex__iterator.html" title="Struct template regex_iterator"><span class="identifier">sregex_iterator</span></a> <span class="identifier">end</span><span class="special">;</span>

<span class="comment">// step through all the matches, and fill in</span>
<span class="comment">// the result map</span>
<span class="keyword">while</span><span class="special">(</span><span class="identifier">it</span> <span class="special">!=</span> <span class="identifier">end</span><span class="special">)</span>
    <span class="special">++</span><span class="identifier">it</span><span class="special">;</span>

<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span><span class="special">[</span><span class="string">"aaa"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span><span class="special">[</span><span class="string">"bbb"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">result</span><span class="special">[</span><span class="string">"ccc"</span><span class="special">]</span> <span class="special">&lt;&lt;</span> <span class="char">'\n'</span><span class="special">;</span>
</pre>
<p> The above code displays: </p>
<pre class="programlisting"><span class="number">1</span>
<span class="number">23</span>
<span class="number">456</span>
</pre>
<p> </p>
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<td><p><span class="term">Parameters:</span></p></td>
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<td><p><span class="term"><code class="computeroutput">args</code></span></p></td>
<td><p>A set of argument bindings, where each argument binding is an assignment expression, the left hand side of which must be an instance of <code class="computeroutput">placeholder&lt;X&gt;</code> for some <code class="computeroutput">X</code>, and the right hand side is an lvalue of type <code class="computeroutput">X</code>.</p></td>
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